A subdomain is a part of a web address that's under the main domain name, such as name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is just "example.com". Every subdomain can have its own site and records and can even be hosted with a different company if you want to use a specific feature that's not provided by your current provider. An example for using a subdomain is if you have a company website and an online store under a subdomain where clients can purchase your products. Additionally, you can have a forum where they can discuss the products and by using subdomains rather than subfolders you will avoid any probability of all sites going down if you perform maintenance, or update one of the website scripts. Keeping the websites separated is also more secure in case of a script security breach.